Method for the preparation of oligomeric alkylene terephthalates

Abstract

Oligomeric alkylene terephthalates having an average degree of condensation of 4 to 7 are produced by passing dimethylterephthalate and diol having at least 3 carbon atoms downwardly through a series of reaction chambers, each outfitted with stirring and heating means employed so that the temperature increases from stage to stage. Thereby transeterification is effected. The chambers are interconnected so that the methanol evolved in each chamber is vented without being admixed with the material undergoing transesterification in any of the other chambers. The transesterification product is then condensed at elevated temperature and under vacuum to produce the oligomer. The oligomers are high reactivity so that the further condensation thereof to polyesters is simplified.

What is claimed is: 
     
       1. Process for production of oligomeric alkylene terephthalates having an average degree of condensation of 4 to 7, a CH 3  O content of less than 0.05 wt.%, and a monomeric terephthalic acid dialkanediol ester content of up to 30 wt.%, by transesterification of dimethylterephthalate and diol of 3 to 8 carbon atoms and condensation of the transesterification product, which comprises: (a) continuously passing the dimethylterephthalate, the diol, and a catalyst for the transesterification through a series of superposed reaction chambers from the uppermost chamber serially to the lowermost chamber, for the transesterification, in liquid phase, methanol being formed in the transesterification reaction and being distilled in the reaction chambers from the liquid phase during the transesterification, and withdrawing the methanol distilled from each chamber without intermixing thereof with the liquid present in any of the other chambers,   (b) the molar proportion of dimethylterephthalate to diol introduced into the reaction chambers being 1:1.1 to 1:1.5,   (c) heating said materials in the reaction chambers so that the temperature in the reaction chambers is in the range of 130°-245° C. and increases stepwise from chamber to chamber, and maintaining the pressure at normal or above normal in said reaction chambers, and stirring the materials during the reaction thereof with stirrers,   (d) passing the transesterification product of the reaction chambers through a vacuum zone maintained at a reduced pressure of 20-300 Torr and temperature of 180°-250° C. for expelling of diol and production of said oligomer and withdrawing the oligmer from the vacuum zone, the temperature in the vacuum zone being higher than the highest temperature in the reaction chambers.   
     
     
       2. Process of claim 1, wherein the proportion of dimethylterephthalate to diol is 1:1.2 to 1:1.4, the pressure in said reaction chambers is normal pressure, and the oligomer is continuously withdrawn from the vacuum zone. 
     
     
       3. Process of claim 1, wherein the catalyst is introduced into the reaction chambers separately from the dimethylterephthalate and diol. 
     
     
       4. Process of claim 1, wherein the catalyst in solution in the diol is introduced into the reaction chambers. 
     
     
       5. Process of claim 1, wherein the catalyst is tetraalkyltitanate. 
     
     
       6. Process of claim 1, wherein the diol is 1,4-butanediol. 
     
     
       7. Process of claim 1, wherein the transesterification product is continuously introduced into the vacuum zone and the oligomerization is performed continuously. 
     
     
       8. Process of claim 1, wherein the oligomer is withdrawn as liquid from the vacuum zone and the liquid is cooled for solidification thereof. 
     
     
       9. Process of claim 1, wherein diol expelled in the vacuum zone is used in the transesterification reaction. 
     
     
       10. Process of claim 1, wherein the pressure in the reaction chambers is a maximum of 4 bars. 
     
     
       11. Process of claim 1, wherein the oligomeric alkylene terephthalate product has a free carboxyl group content of less than 30 m.eq. per kilogram. 
     
     
       12. Process of claim 1, wherein the content of monomeric terephthalic acid dialkanediol ester in the oligomer is 0.5 to 20 wt%. 
     
     
       13. Process of claim 1, wherein diol withdrawn from the reaction chambers is recovered and recycled to the reaction chambers.